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Keeping an eye on all the players: The combination of bulky phosphonium tags and pressurized sample infusion transforms electrospray ionization mass spectrometry into a tool capable of producing dense data on the relative concentrations of all components of a catalytic reaction, such as the palladium‐catalyzed coupling of an aryl iodide with phenylacetylene (see graph).

Real-time monitoring of the Suzuki–Miyaura reaction using mass spectrometry during sequential addition of the various reaction components suggests that a dynamic series of equilibria exist in these solutions. Depending on conditions, the boronic acid can be dehydrated, deprotonated, hydroxylated (or alkoxylated), or fluorinated. Palladium–phosphine species present include Pd(0) (to which the aryl iodide rapidly oxidatively adds), the Pd­(II) aryl iodide complex, a cationic Pd­(II) species formed by dissociation of the iodide ligand, and the Pd­(II) bisaryl complex that ultimately extrudes the product through reductive elimination. No fluorinated or hydr­(alk-)­oxylated palladium complexes were observed under catalytic conditions. Several transmetalation combinations were excluded as reactive partners, but several possibilities remain, and more than one mechanism is likely to be operative, even under similar conditions.

The rate of hydrodehalogenation of aryl iodides with a palladium catalyst in methanol exhibits a strong primary kinetic isotope effect from both CD3OD and CH3OD, suggesting that deprotonation plays a major role in the mechanism.

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